The present invention relates to a data receiver and method for reducing the effect of feedback from an output signal of the receiver to an input to the receiver.
Infrared wireless data communication is a useful method for short range (in the approximate range of 0-10 meters) wireless transfer of data between electronic equipment; such as, cellular phones, computers, computer peripherals (printers, modems, keyboards, cursor control devices, etc.), electronic keys, electronic ID devices, and network equipment. Infrared wireless communication devices typically have the advantages of smaller size, lower cost, fewer regulatory requirements, and a well defined transmission coverage area as compared to radio frequency wireless technology (i.e. the zone of transmission is bounded by physical walls and therefore more useful in an office environment). In addition, infrared wireless communication has further advantages with regard to reliability, electromagnetic compatibility, multiplexing capability, easier mechanical design, and convenience to the user as compared to cable based communication technology. As a result, infrared data communication devices are useful for replacing 0-10 meter long data transfer cables between electronic devices, provided that their size and costs can be reduced to that of comparable cable technology.
Infrared data communications devices typically consist of transmitter and receiver components. The infrared data transmitter section consists of one or more infrared light emitting diodes (LEDs), an infrared lens, and an LED current driver. A conventional infrared data receiver typically consists of an infrared photodiode and a high gain receiver amplifier with various signal processing functions, such as automatic gain control (AGC), background current cancelling, filtering, and demodulation. For one-directional data transfer, only a transmitter at the originating end and a receiver at the answering end is required. For bidirectional communication, a receiver and transmitter at each end is required. A combined transmitter and receiver is called a transceiver.
In typical high volume applications, it is now standard practice to fabricate the receiver circuitry and transmitter driver in a single integrated circuit (IC) to produce a transceiver IC. In turn, a transceiver IC, infrared photodiode and LED along with lenses for the photodiode and LED are assembled together in a plastic molded package designed to be small in size and allow placement in the incorporating electronic device so as to have a wide angle of view (typically through an infrared window on its case). The transceiver IC is designed to digitally interface to some type of serial data communications device such as an Infrared Communication Controller (ICC), UART, USART, or a microprocessor performing the same function.
A representative example of a conventional infrared data transmitter and receiver pair is shown in FIG. 1. Infrared transmitter 10 includes LED 16 which generates a modulated infrared pulse in response to transistor 14 being driven by the input data signal D.sub.IN. The modulated infrared signal is optically coupled to an infrared detector, such as photodiode 24 normally operated in current mode (versus voltage mode) producing an output current which is a linear analog of the optical infrared signal falling on it. The infrared pulses generated by LED 16 strike photodiode 24 causing it to conduct current responsive to the transmitted input data signal D.sub.IN thereby generating a received data signal at D.sub.IR.
Data can be modulated on the infrared transmitted signal by any of a number of well known methods. Two of the most popular methods are defined by the Infrared Data Association (IrDA) and Sharp corporation (Sharp ASK). IrDA Physical Layer Link Specification 1.1e specifies two main physical layer infrared modulation methods. One method is a low-speed (2 Kbp/s to 1.15 Mbp/s) on-off infrared carrier asynchronous modulation where the presence of a pulse indicates a 0 bit and the absence of a pulse indicates a 1 bit. The second method is a high speed (4 Mb/s) synchronous Four Pulse Position Modulation (4PPM) method in which the time position of a 125 ns infrared pulse in a 500 ns frame encodes two bits of information. The Sharp ASK method is similar to the low speed IrDA method but also modulates the infrared carrier with a 500 Khz signal to facilitate differentiating between a valid signal and ambient infrared signals.
In receiver 20, the received signal at D.sub.IR is transformed into a voltage signal and amplified by amplifier 26. The signal output from amplifier 26 then feeds into comparator 42 which demodulates the received signal by comparing it to a detection threshold voltage V.sub.DET in order to produce a digital output data signal at D.sub.OUT.
The received signal waveform will have edges with slope and will often include a superimposed noise signal. As a result, V.sub.DET is ideally placed at the center of the received signal waveform so that the output data signal has a consistent waveform width despite the slope of the received signal edges. Also, placing V.sub.DET at the center of the received signal improves the noise immunity of receiver 20 because the voltage difference between V.sub.DET and both the high and low levels of the received signal is maximized such that noise peaks are less likely to result in spurious transitions in D.sub.OUT.
The received signal, however, can vary in amplitude by several orders of magnitude due primarily to variations in the distance between transmitter 10 and receiver 20. The strength of the received signal decreases proportional to the square of the distance. Depending on the range and intensity of the infrared transmitter, the photodiode outputs signal current in the range of 5 na to 5 ma plus DC and AC currents arising from ambient infrared sources of sunlight, incandescent and florescent lighting. As a consequence, the center of the received signal waveform will vary, whereas V.sub.DET must generally be maintained at a constant level. To address this problem, receivers typically include an automatic gain control mechanism to adjust the gain responsive to the received signal amplitude. The received signal is fed to AGC peak detector 36 which amplifies the signal and drives current through diode 32 into capacitor 28 when the signal exceeds the AGC threshold voltage V.sub.AGC in order to generate a gain control signal. The gain control signal increases in response to increasing signal strength and correspondingly reduces the gain of amplifier 26 so that the amplitude of the received signal at the output of amplifier 26 remains relatively constant despite variations in received signal strength.
At a minimum, infrared receiver 20 amplifies the photodetector signal current and then level detects or demodulates the signal when it rises above the detect threshold V.sub.DET thereby producing a digital output pulse at D.sub.OUT. For improved performance, the receiver may also perform the added functions of blocking or correcting DC and low frequency AC ambient (1-300 ua) signals and Automatic Gain Control (AGC) which improves both noise immunity and minimizes output pulse width variation with signal strength.
Because the typical infrared photodiode has a very small area its output current is low, typically 10 na to 200 na at ranges of 1 to 3 meters when receiving signals from an infrared LED driven by current pulses of 80 us to 125 ns, at levels of 50 ma to 500 ma. There are well known electronic methods for low noise amplification of such weak, wideband signals. However, a common problem of such amplification systems is significant disruptive feedback from the receiver output D.sub.OUT to the photodiode input D.sub.IR. Others have recognized that feedback exists but have attributed it to substrate coupling and capacitive loading due to MIS capacitor parasitics (Ritter et al, "Circuit and System Challenges in IR Wireless Communication" ISSCC96, Session 25, TD: Optical Interconnects, High Temp., & Packaging, Paper 25.1). However, empirical observation revealed that the largest feedback mechanism is due to electrostatic coupling (capacitive), represented by capacitor 40 in FIG. 1, between the data output pin D.sub.OUT and received data input pin D.sub.IR. Significant coupling can also occur on ground, power, and bias lines of receiver 20.
As an example, because an IC infrared receiver is preferably small, there is very close proximity (less than 1 cm.) between the input D.sub.IR from photodiode 24 and the receiver output, the output of comparator 42. Consequently, in infrared receiver 20 the electrostatic coupling (feedback) between these two signal paths is typically from 5 to 50 femto farads (fF) capacitance (capacitor 40). In addition, there may be added capacitive coupling to the photodiode input D.sub.IR from the receiver output signal trace on the circuit board on which the receiver package is mounted. During receiver signal detection and subsequent output this feedback causes a large input spurious signal that disrupts moderate to weak received signals which may limit operation to distances under 1 meter in order to obtain received signal amplitudes sufficient to overcome the feedback signal.
The disruptive effect of feedback varies. For out of phase feedback, or where the receiver response has transient overshoot or ringing, then output pulse oscillations are likely to occur and may last beyond the duration of the input pulse, see FIGS. 2A-2C. Even if pulse oscillations do not occur on systems using AGC, the feedback transient may cause receiver gain reduction after some delay so that pulse detection drops occur.
Generally, to prevent significant disruption of the received signal it is desirable that the amplitude of the feedback or noise be at least 12 db less (1/4) than the minimum amplitude of the received signal. Since the feedback signal is a narrow current transient arising from the fast differentiated edge (10-200 ns rise and fall time) of an output pulse in D.sub.OUT, the feedback, after bandwidth filtering, appears at the receiver detect comparator 42 as an exponentially decaying pulse with a time constant equal to the receiver bandwidth time constant (which for good pulse fidelity is typically at least 1/2 the signal pulse width).
For example, if infrared receiver which has a typical value of 10 fF of feedback capacitance 40 and the voltage swing D.sub.OUT is 5V, then a 50 femto couloumb charge transient will be transferred to D.sub.IR on each transition of D.sub.OUT. On a receiver designed to receive a 1.66 us pulse with good fidelity, this transient will appear as a 0.83 us, 60 na (50 fcoul) disrupting signal. On such a receiver, the input signal detect threshold V.sub.DET would need to be set at a level corresponding to a 240 nA current signal at the input D.sub.IR (approximately 240 mV in the present example) to avoid having the input signal be disrupted by feedback.
Another problem with the signal disruption of feedback is that if feedback exceeds the detect threshold, V.sub.DET, then operation of receiver 20 even with a strong received signal level may fail. Typically this occurs because although a strong signal may overpower feedback from the leading edge of the transition on D.sub.OUT, the trailing edge feedback may cause oscillations or extra pulses because it occurs after the trailing edge of the pulse in the received signal. Consequently, the V.sub.DET on infrared receiver 20 needs to be set to a higher lever (1.5-3 times) than is otherwise necessary to ensure that feedback does not cause receiver failure due to manufacturing variances or feedback variances due to the mounting circuit board trace layout. In the above example, to provide this variance margin, the detect threshold, V.sub.DET, would need to be set to a reference level corresponding to an undesirably high input current level of 360 na to 720 na. With typical LED transmitters, this would result in a maximum receiving range of 1/2 meter or less.
Low cost, small photodetectors produce 10 na-100 na of output in the typical operation range of 1-3 meters when illuminated by IrDA specified infrared transmitters. The feedback signal due to parasitic capacitance 40 will often be 20 to 30 dB greater in strength than the minimum received signal from the photodiode. Therefore, it is desirable to reduce the effects of capacitive feedback from the receiver output to photodetector input D.sub.IR by 20 to 40 db (0.1 to 0.01 times).
One conventional solution for reducing the magnitude of capacitive feedback coupling is to use electrostatic shielding. A shield is a conductor terminated at local signal ground and placed close to or between the radiating and receiving signal conductors. A typical shield for moderate feedback attenuation (20 db reduction) is a proximity ground plane, while a typical shield for higher feedback attenuation (40 db reduction) is an enclosing conductive box with access holes for conductors and optical windows. Although the shield works well, it adds manufacturing cost and increases the size of a receiver or transceiver module.
Another conventional solution is to use a larger photodiode for photodetector 24. Although the increased area of the diode will tend to increase feedback capacitance the received signal will generally be increased by a greater amount than the feedback. However, such a large photodetector (having at least 10 times the active area of a small diode) is significantly more expensive than a small photodetector, and a large photodetector or photo transistor will not fit in a desirable small module.
Yet another known solution is to use differential input and output techniques as illustrated in FIGS. 3A and 3B. A differential input amplifier can reject common mode noise or feedback by over 40 db assuming the feedback noise is equal on both inputs of the differential input amplifier. However, this is typically not a valid assumption in an actual infrared receiver. This is because a single photodiode coupled across the inputs of the differential input amplifier will have significantly different feedback coupling capacitances due to the inherent asymmetric geometry of the two photodiode connections. Consequently, using a single photodiode with a differential input amplifier will typically not improve performance over a receiver with a non-differential amplifier.
Another differential solution is to use two receiver outputs that are 180.degree. out of phase with one another, as illustrated in FIG. 3C. If they are placed symmetrically with respect to the photodiode input then each output tends to cancel the feedback due to the other output and feedback coupling can be reduced by a factor of ten. Although this solution is less expensive than the dual diode differential technique, it still suffers from the same undesirable layout symmetry constraints.
A better performing differential input amplifier method is to use two matched photodiodes with identical mounting geometries wherein one photodiode receives the transmitted infrared signal and the other photodiode has its light path covered, as shown in FIG. 3D. Although this differential solution can reduce feedback by an order of ten using the added diode, it is more expensive, larger in size and has the added mechanical constraint that the diode geometry needs to be symmetrical with respect to the receiver output conductor connection. In addition, the diode feedback symmetry may be disrupted due to asymmetric shielding effects of nearby conductors on the circuit board on which the receiver is mounted.
Accordingly, there remains a need for a method for addressing feedback in an infrared receiver to improve performance, but without the drawbacks of the conventional solutions.